1. Field of the Invention
The present invention relates to a corpuscular-beam apparatus including a specimen holder which has at least two degrees of freedom of translation and at least one degree of freedom of rotation, and a control means which corrects the translational coordinates of the specimen holder when the specimen holder is rotated so that a predetermined specimen point retains its position in the apparatus. (Rotation of a specimen point means, in the context of the present invention, a turning, i.e., a circular motion or rotation about an axis disposed perpendicular to the plane of the specimen, as well as tilting, i.e., a circular motion or rotation about an axis in the specimen.)
2. Description of the Prior Art
A corpuscular-beam apparatus of the above-described type may comprise an electron microscope. See, for example, U.S. Pat. No. 3,727,051. In such a microscope, the specimen holder comprises a goniometer, in particular a double-tilt goniometer, in which the specimen is suspended in gimbals in a frame which is movable in a plane disposed perpendicular to the electron beam of the microscope and has two tilt axes. The first of the two axes extends in the direction of one translational coordinate axis. The second axis is disposed perpendicular to the first axis and is rotatable about that axis. The described goniometer permits a specimen to be moved with four degrees of freedom. The fifth degree of freedom, corresponding to the third translational coordinate (in the direction of the optical axis of the microscope), is simulated by changing the focal length of the objective lens.
Such electron microscopes also include a control unit including a computer into which the instantaneous values of the translational coordinates and the tilt angle are entered. The computer calculates the corrections of the specimen position required in the event of a tilt, controls the translational drives, and changes the focal length of the objective lens of the microscope as required. The corrections are exact calculations which require considerable computation. Thus, it is necessary, for example, to generate electric voltage signals which are proportional to a product of four trigonometric functions of the specimen tilt angles.
Aside from the double-tilt goniometer described above, there are also tilt-rotation goniometers, such as that described in French Pat. No. 2,102,553, which are known in the art. In these specimen holders, the specimen is supported on a rotary table which is held in a frame movable in a plane perpendicular to the microscope beam and which is tiltable about an axis extending in the direction of one coordinate axis. If control of the correction of the translational coordinates is desired in such a goniometer in the event of a rotation of the specimen holder in the exact manner described, it is necessary to again calculate products of trigonometric functions. The effort required to do this is obviously excessive.